The invention relates to multilayer films in the form of tubing, in particular fiber-reinforced films, which are based on cellulose hydrate and have on the inside a film coating which is impermeable to water and water vapor. The inside coating comprises a film of a synthetic thermoplastic polymer, in particular a copolymer. On the outside the tubing has a permeable layer based on a synthetic elastic copolymer. The invention also relates to processes for the manufacture of skins in the form of tubing, according to the invention, as well as to the use of the said films in the form of tubing as a packaging material, in particular for use as an artificial sausage casing.
Large quantities of cellulose hydrate tubing, having on the inside a film coating which is impermeable to water vapor and consists of a synthetic thermoplastic vinylidene chloride copolymer, are used as artificial sausage casings and are suitable for this purpose because, as a result of extensive shrinkage in the course of their intended use as sausage casings, they are able to fit well around the filling material and thus always yield sausages, the skin of which is in tight contact with the filling material and which thus gives the sausages an attractive appearance.
This advantage of the known artificial sausage casings is at the same time accompanied by a serious defect, namely that artificial casings for sausages are prone to damage by tearing and shock.
U.S. Pat. No. 4,002,712 describes tubing based on cellulose hydrate modified by chemical agents, which tubing is suitable for use as an artificial sausage casing and which, even if its content of water-extractable chemical plasticizer is lost, still has the suppleness which a tubing of cellulose hydrate possesses. Moreover, a proposal which does not yet form part of the state of the art (German Patent Application No. P 26 54 427.4) describes tubing which is suitable for use as an artificial sausage casing and which has been partially plasticized by means of so-called permanent chemical plasticizers, so that, even after its content of water-soluble chemical plasticizer has been lost, it has the suppleness of tubing of cellulose hydrate, or possesses an even better suppleness.
A further proposal which does not yet form part of the state of the art describes tubing based on cellulose hydrate, which is suitable for use as artificial sausage casings and which contains chemical compounds having N-methylolcarbamate end groups in a firm chemical bond. This tubing which has been described likewise retains a good suppleness after loss of the extractable secondary chemical plasticizer which, as a result of the manufacturing process of the tubing, is contained in the wall thereof.
The quantitative loss of water-extractable chemical plasticizer during the intended use of the multilayer film material as artificial sausage casings, for example, during the process of boiling the sausages in hot water, initiates a shrinkage of the casings by about 12 to 15%. As a result, a high internal pressure develops in the filling material cavity of the sausage casing, with a simultaneous embrittlement of the cellulose hydrate layer thereof. This layer is separated on the inside of the tubing from the sausage material which contains moisture by the film coating, which is impermeable to water vapor. This has the consequence of an undesirable extensive tearing and tear propagation of the sausage casings when the sausages are cut open, the sausage material being squeezed out of the cutting surface in an undesirable manner. The sensitivity of sausages of this type to shocks in transit is a further disadvantage inherent in the known and described artificial sausage casings which are based on cellulose hydrate and have on their inside a film coating which is impermeable to water vapor.
Admittedly, it is possible to partially avoid these disadvantages by using the tubing described in the aforesaid U.S. patent or tubing according to the descriptions which do not yet form part of the state of the art. However, in order to eliminate these disadvantages altogether, it would be necessary for the said tubing to contain such a large amount of non-extractable chemical plasticizer that, as a result, an undesirable deterioration of the mechanical properties of the tubing would necessarily result.